1. Field of the Art
This invention relates generally to molded plastic optical components like plastic lenses, and more particularly to molded plastic optical components which are stripped of gate stubs, which remain on the molded optical components after gate cutting, and a method for machining outer rims of molded optical components to get rid of gate stubs therefrom.
2. Prior Art
Plastic optical components including plastic lenses are used on a diversity of optical appliances. For instance, plastic objective lenses are used on optical pickup devices of various optical disc drives. Objective lens of this sort are usually fitted in lens holders of pickup devices and thereby retained correctly in an optically aligned position. As means for correctly positioning a lens component within a lens holder, it has been the general practice to provide an outer rim portion around the circumference of each lens component. For facilitating the lens mounting correctly in a predetermined position on or within a lens holder, the outer rim portion is formed in an annular or cylindrical shape which can be easily fitted in an optically aligned position within a lens holder.
The lens portion and the outer rim portion of an optical component are preferred to be formed as one integral body, and, for this purpose, plastic injection molding is widely resorted to in the manufacture of optical components of this sort. The mold for use in such plastic injection molding process is provided with a cavity of predetermined shape including transfer surfaces which define the surface contour of a lens to be molded. Molten synthetic resin material injected into the mold is cured to shape within the mold cavity to obtain moldings with lens surfaces copied from the transfer surfaces of the mold cavity. Molten synthetic resin material is injected into the mold cavity through a sprue which is provided in a mold assembly as a resin feed passage. The sprue is connected to the mold cavity through a gate or a constricted passage in order to fill the resin material in the entire mold cavity.
In the injection molding process, surplus resin material remains and cures in the sprue and gate which is connected to the mold cavity, so that a molding operation is usually followed by the so-called gate-cut operation to cut off the gate or the surplus resin material from the molded products of optical components. Of course, the gate cut can be carried out within the mold in case a movable cutter member is operatively provided within a mold assembly. However, since the provision of a movable cutter within a mold will lead to complication of the mold construction or other problems, it is desirable to carry out the gate cut on a molded optical component after ejection from a mold. After cutting off the surplus resin material or the gate in an initial stage of gate cut, however, there still remains a gate stub which more or less projects radially outward from the circumference of the outer rim portion of each molded optical component. Therefore, in a nest or final stage of gate cut, the gate stub has to be removed from each molded product by the use of a cutting tool.
This final stage of gate cut, which is directed to the removal of gate stubs bears a great influence on the quality of final products as optical components.
Ultimately, each one of molded optical components has to be mounted accurately in an optically aligned position on a holder means no matter whether it is a lens holder on an optical pickup of an optical disc drive or of other optical appliances. For this purpose, a reference surface which matches a reference surface on the part of a lens holder is formed on each optical components during the molding process. Normally, a reference surface of this sort is formed on an outer rim portion of each optical component, more particularly, on one end face and an adjoining circumferential surface of an outer rim portion. On the circumferential side, the reference surface is normally provided to cover part of the axial length of the outer rim portion contiguously from one end face of the latter, instead of covering the entire axial length of the outer rim, and in such a position as to evade a gate-cut portions on the circumference of the outer rim. Should the reference surface be damaged by a cutter blade in a gate-cut operation, the resulting surface flaw could make it difficult to assemble the optical component into an optically aligned position on a lens holder. Therefore, in the final stage of gate cut, namely, in the stage of removing a gate stub on the circumference of the outer rim portion of a molded optical component, a cutting tool should be kept out of contact with the reference surface on the circumference of the outer rim portion.
The gate-cut portion on the circumference of an outer rim of a molded optical component basically has nothing to do with the positioning of optical element on a lens holder, so that, with a lens holder of a certain shape, the existence of a small gate stub or surface ruggedness resulting from an incomplete gate cut may not give rise to problems in particular when mounting the optical component on a lens holder later on. In this connection, however, it is necessary to take into consideration that, for the convenience of handling in a subsequent inspection stage or in an actual mounting stage, a large number of molded optical components are usually accommodated in a jig in the form of a tubular stick. Further, since the gate cut is usually restricted to a minimum necessary amount for the purpose of preventing the reference surface from being damaged by a cutting tool as mentioned above, it is very likely that surface condition of the gate-cut portions vary between individual molded components and relatively large projections remain on some optical components as gate stubs. Because of irregularities in surface ruggedness at gate-cut portions, in some cases it becomes difficult to insert molded optical components into a jig stick, and in the worst case the insertion itself is impossible. If forcibly pushed into a stick, there will occur jamming troubles at the time of discharging optical components out of the jig stick, making it difficult to mount the optical components automatically by the use of a robot machine. Nevertheless, the moldings with such incomplete gate-cut portions are not necessarily defective in optical quality, and are usable after corrective machining of the incomplete gate-cut portions.
For the reasons as stated above, high precision gate cutting would require to position each molded optical component precisely relative to a cutting tool in an extremely strict manner. Besides, because of annular ring-like shape of the outer rim portion, there has been another problem that strict control of positions and movements of cutting tool and optical components is possible only by the use of an apparatus of relatively large-scale involving complicate control mechanisms.